As mentioned in T12295, there are two kinds of recurring events we use that we currently cannot schedule in Calendar:

• Repeat on $nth $weekday of the month. For example, some teams hold meetings on the first Monday of the month. Cross-project meetings are on the second Tuesday of the month.
• Repeat on $weekday every $x weeks. For example, a sprint retrospective every two weeks.

You can import these kinds of events using an ICS file, but then they'll be read-only and you won't be able to add comments.